Docs: Streaming timeout with HttpClient

[JamesNK]

See https://github.com/protobuf-net/protobuf-net.Grpc/issues/210

When a channel is customized to use a HttpClient and has log running streaming calls then HttpClient.Timeout will be triggered. Add section to troubleshooting doc.

[Drawaes]

I recently hit this issue and found this and the protobuf-net info after a lot of searching, reverting to just pushing in a handler instead of a whole http client was the fix for me also so a documentation note would be great.